High speed multi-touch touch device and controller therefor

ABSTRACT

A touch-sensitive device includes a touch panel, a drive unit, a sense unit, and a measurement unit. A touch applied to a node of the panel changes a capacitive coupling between two electrodes (a drive electrode and a sense electrode) of the touch panel. The drive unit delivers a drive signal, which may comprise one or more drive pulses, to the drive electrode. The sense unit couples to the sense electrode, and generates a response signal that includes a differentiated representation of the drive signal. The amplitude of the response signal is responsive to the capacitive coupling between the electrodes, and is measured to provide an indication of a touch at the node.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional patentapplication No. 61/182,366, filed May 29, 2009, and U.S. Provisionalpatent application No. 61/231,471, filed Aug. 5, 2009, the disclosure ofwhich is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

This invention relates generally to touch-sensitive devices,particularly those that rely on a capacitive coupling between a user'sfinger or other touch implement and the touch device, with particularapplication to such devices that are capable of detecting multipletouches applied to different portions of the touch device at the sametime.

BACKGROUND

Touch sensitive devices allow a user to conveniently interface withelectronic systems and displays by reducing or eliminating the need formechanical buttons, keypads, keyboards, and pointing devices. Forexample, a user can carry out a complicated sequence of instructions bysimply touching an on-display touch screen at a location identified byan icon.

There are several types of technologies for implementing a touchsensitive device including, for example, resistive, infrared,capacitive, surface acoustic wave, electromagnetic, near field imaging,etc. Capacitive touch sensing devices have been found to work well in anumber of applications. In many touch sensitive devices, the input issensed when a conductive object in the sensor is capacitively coupled toa conductive touch implement such as a user's finger. Generally,whenever two electrically conductive members come into proximity withone another without actually touching, a capacitance is formedtherebetween. In the case of a capacitive touch sensitive device, as anobject such as a finger approaches the touch sensing surface, a tinycapacitance forms between the object and the sensing points in closeproximity to the object. By detecting changes in capacitance at each ofthe sensing points and noting the position of the sensing points, thesensing circuit can recognize multiple objects and determine thecharacteristics of the object as it is moved across the touch surface.

There are two known techniques used to capacitively measure touch. Thefirst is to measure capacitance-to-ground, whereby a signal is appliedto an electrode. A touch in proximity to the electrode causes signalcurrent to flow from the electrode, through an object such as a finger,to electrical ground.

The second technique used to capacitively measure touch is throughmutual capacitance. Mutual capacitance touch screens apply a signal to adriven electrode, which is capacitively coupled to a receiver electrodeby an electric field. Signal coupling between the two electrodes isreduced by an object in proximity, which reduces the capacitivecoupling.

Within the context of the second technique, various additionaltechniques have been used to measure the mutual capacitance betweenelectrodes. In one such technique, a capacitor coupled to a receiverelectrode is used to accumulate multiple charges associated withmultiple pulses of a drive signal. Each pulse of the drive signal thuscontributes only a small portion of the total charge built up on this“integrating capacitor”. Reference is made to U.S. Pat. No. 6,452,514(Philipp). This technique has good noise immunity, but its speed may belimited depending upon the number of pulses needed to charge theintegrating capacitor.

BRIEF SUMMARY

The present application discloses, inter alia, touch-sensitive devicescapable of detecting multiple touches applied to different portions ofthe touch device at the same time or at overlapping times. Moreover, thetouch devices need not employ an integrating capacitor in order tomeasure the capacitive coupling between the drive electrodes and thereceive electrodes. Rather, in at least some embodiments, a single pulsefrom a drive signal may be all that is necessary to measure thecapacitive coupling between a particular drive electrode and aparticular receive electrode, or even between a particular driveelectrode and a large plurality of (e.g. all of the) receive electrodes.To accomplish this, assuming a suitable pulse shape is used for thedrive signal, differentiation circuits are preferably coupled to thereceive electrodes so that a differentiated representation of the drivesignal, referred to as a response signal, is generated for each receiveelectrode. In an exemplary embodiment, each differentiation circuit maycomprise an operational amplifier (op amp) with a feedback resistorconnected between an inverting input of the op amp and the output of theop amp, with the inverting input also being connected to a given receiveelectrode. Other known differentiation circuit designs can also be used,so long as the circuit provides an output that includes in some form atleast an approximation of the derivative with respect to time of thedrive signal.

A characteristic amplitude, such as a peak amplitude or averageamplitude, of the response signal is indicative of the capacitivecoupling between the drive electrode and the receive electrode beingsampled. A touch at the node corresponding to the particular drive andreceive electrodes has the effect of reducing capacitive coupling andreducing the characteristic amplitude. Such a reduction in amplitude canbe measured even with only a single pulse of the drive signal. Multipletouches at different portions of the touch device that are simultaneousor that otherwise overlap in time can be detected in this manner. Ifnoise reduction is desired, a selected number of multiple pulses from adrive signal may be employed for each drive/receive electrode pair(i.e., node), and the amplitude measurements measured or otherwiseprocessed to provide a lower noise measurement.

The application also discloses touch-sensitive apparatuses that includea touch panel, a drive unit, a sense unit, and a measurement unit. Thepanel may include a touch surface and a plurality of electrodes definingan electrode matrix, the plurality of electrodes including a pluralityof drive electrodes and a plurality of receive electrodes. Each driveelectrode is capacitively coupled to each receive electrode at arespective node of the matrix. The panel is configured such that a touchon the touch surface proximate a given one of the nodes changes acoupling capacitance between the drive electrode and the receiveelectrode associated with the given node. The drive unit, in turn, isconfigured to generate a drive signal and to deliver the drive signal tothe drive electrodes one at a time, e.g. through a multiplexer. Thedrive signal may be or include only one individual drive pulse, or itmay include a plurality or train of such drive pulses. The sense unitmay be configured to generate, for each drive signal delivered to eachdrive electrode, response signals for the plurality of receiveelectrodes that are capacitively coupled to such drive electrode, eachof the response signals including a differentiated representation of thedrive signal. An amplitude of each of these response signals isresponsive to the coupling capacitance at the associated node. Finally,the measurement unit is preferably configured to measure the amplitudeof each of the response signals for each of the nodes, and to determinetherefrom the positions of multiple temporally overlapping touches, ifpresent, on the touch surface.

The shape of the drive pulse(s) used in the drive signal may be tailoredor selected so as to provide a desired waveform shape for the responsesignals. For example, if a rectangle shape is used for the drive pulse,the response signal generated by the sense unit typically comprises apair of opposite polarity impulse pulses, the peak amplitude of whichcan be isolated with a peak detector and optional sample/hold buffer.Alternatively, if a ramp-shaped drive pulse is selected, the responsesignal typically comprises a pulse shape that is nominally rectangular,i.e., it includes a relatively constant amplitude plateau disposedbetween two relatively steep high-to-low transitions, examples of whichare described below. Such a rectangular-shaped response signal allowsfor the possible elimination of certain circuit elements, and overallsimplification of the touch device, as described further below.

The application also discloses touch-sensitive apparatuses that includea touch panel, a drive unit, and a sense unit. The panel includes atouch surface and a plurality of electrodes defining an electrodematrix, the electrode matrix being configured such that a touch on thetouch surface proximate a given node of the matrix changes a couplingcapacitance between two of the electrodes. The drive unit is coupled tothe electrode matrix and configured to generate a drive signal thatincludes one or more ramped pulses. The sense unit is also coupled tothe electrode matrix, and is configured to generate, in response to thedrive signal, at least one response signal that includes one or morerectangle pulses, an amplitude of the at least one response signal beingresponsive to a touch on the touch surface.

Related methods, systems, and articles are also discussed.

These and other aspects of the present application will be apparent fromthe detailed description below. In no event, however, should the abovesummaries be construed as limitations on the claimed subject matter,which subject matter is defined solely by the attached claims, as may beamended during prosecution.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view of a touch device;

FIG. 2 is a schematic side view of a portion of a touch panel used in atouch device;

FIG. 3 a is a schematic view of a touch device in which relevant driveand detection circuitry is shown in the context of one drive electrodeand one receive electrode capacitively coupled thereto;

FIG. 3 b is a schematic view of a touch sensitive device similar to thatof FIG. 3 a, but including additional circuitry to account fordifferences of signal strength on receiver electrodes;

FIG. 3 c is a schematic view of a touch sensitive device similar to thatof FIG. 3 a, but including additional circuitry to account for noisefrom, for example, a display;

FIG. 4 a is a graph of a drive signal and a corresponding (modeled)response signal for the touch device of FIG. 3 a, wherein the drivesignal includes rectangle pulses and the response signal includesimpulse pulses;

FIG. 4 b is a graph showing modeled waveforms for three drivenelectrodes, and associated response waveforms on three receiveelectrodes;

FIG. 5 a is a graph similar to that of FIG. 4 a but for a differentdrive signal, the drive signal including ramped pulses and the responsesignal including rectangle-like pulses;

FIG. 5 b is a graph showing modeled waveforms for three drivenelectrodes, and associated response waveforms on three receiveelectrodes, similar to FIG. 4 b;

FIG. 6 a is a graph of still another drive signal and a schematicdepiction of an expected response signal for the touch device of FIG. 3a, the drive signal including ramped pulses and the response signalincluding rectangle pulses;

FIG. 6 b is a graph showing modeled waveforms for three drivenelectrodes, and associated response waveforms on three receiveelectrodes, similar to FIGS. 4 b and 5 b;

FIG. 7 is a graph of a drive signal and corresponding (modeled) responsesignal for the touch device of FIG. 3 c, wherein the drive signalincludes rectangle pulses and the response signal includes impulsepulses; and,

FIG. 8 is a schematic view of a touch device that includes a touch panelhaving a 4×8 matrix of capacitively coupled electrodes, and variouscircuit components that can be used to detect multiple simultaneoustouches on the touch panel.

In the figures, like reference numerals designate like elements.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

In FIG. 1, an exemplary touch device 110 is shown. The device 110includes a touch panel 112 connected to electronic circuitry, which forsimplicity is grouped together into a single schematic box labeled 114and referred to collectively as a controller.

The touch panel 112 is shown as having a 5×5 matrix of column electrodes116 a-e and row electrodes 118 a-e, but other numbers of electrodes andother matrix sizes can also be used. The panel 112 is typicallysubstantially transparent so that the user is able to view an object,such as the pixilated display of a computer, hand-held device, mobilephone, or other peripheral device, through the panel 112. The boundary120 represents the viewing area of the panel 112 and also preferably theviewing area of such a display, if used. The electrodes 116 a-e, 118 a-eare spatially distributed, from a plan view perspective, over theviewing area 120. For ease of illustration the electrodes are shown tobe wide and obtrusive, but in practice they may be relatively narrow andinconspicuous to the user. Further, they may be designed to havevariable widths, e.g., an increased width in the form of a diamond- orother-shaped pad in the vicinity of the nodes of the matrix in order toincrease the inter-electrode fringe field and thereby increase theeffect of a touch on the electrode-to-electrode capacitive coupling. Inexemplary embodiments the electrodes may be composed of indium tin oxide(ITO) or other suitable electrically conductive materials. From a depthperspective, the column electrodes may lie in a different plane than therow electrodes (from the perspective of FIG. 1, the column electrodes116 a-e lie underneath the row electrodes 118 a-e) such that nosignificant ohmic contact is made between column and row electrodes, andso that the only significant electrical coupling between a given columnelectrode and a given row electrode is capacitive coupling. The matrixof electrodes typically lies beneath a cover glass, plastic film, or thelike, so that the electrodes are protected from direct physical contactwith a user's finger or other touch-related implement. An exposedsurface of such a cover glass, film, or the like may be referred to as atouch surface. Additionally, in display-type applications, a back shieldmay be placed between the display and the touch panel 112. Such a backshield typically consists of a conductive ITO coating on a glass orfilm, and can be grounded or driven with a waveform that reduces signalcoupling into touch panel 112 from external electrical interferencesources. Other approaches to back shielding are known in the art. Ingeneral, a back shield reduces noise sensed by touch panel 112, which insome embodiments may provide improved touch sensitivity (e.g., abilityto sense a lighter touch) and faster response time. Back shields aresometimes used in conjunction with other noise reduction approaches,including spacing apart touch panel 112 and a display, as noise strengthfrom LCD displays, for example, rapidly decreases over distance. Inaddition to these techniques, other approaches to dealing with noiseproblems are discussed in reference to various embodiments, below.

The capacitive coupling between a given row and column electrode isprimarily a function of the geometry of the electrodes in the regionwhere the electrodes are closest together. Such regions correspond tothe “nodes” of the electrode matrix, some of which are labeled inFIG. 1. For example, capacitive coupling between column electrode 116 aand row electrode 118 d occurs primarily at node 122, and capacitivecoupling between column electrode 116 b and row electrode 118 e occursprimarily at node 124. The 5×5 matrix of FIG. 1 has 25 such nodes, anyone of which can be addressed by controller 114 via appropriateselection of one of the control lines 126, which individually couple therespective column electrodes 116 a-e to the controller, and appropriateselection of one of the control lines 128, which individually couple therespective row electrodes 118 a-e to the controller.

When a finger 130 of a user or other touch implement comes into contactor near-contact with the touch surface of the device 110, as shown attouch location 131, the finger capacitively couples to the electrodematrix. The finger draws charge from the matrix, particularly from thoseelectrodes lying closest to the touch location, and in doing so itchanges the coupling capacitance between the electrodes corresponding tothe nearest node(s). For example, the touch at touch location 131 liesnearest the node corresponding to electrodes 116 c/118 b. As describedfurther below, this change in coupling capacitance can be detected bycontroller 114 and interpreted as a touch at or near the 116 a/118 bnode. Preferably, the controller is configured to rapidly detect thechange in capacitance, if any, of all of the nodes of the matrix, and iscapable of analyzing the magnitudes of capacitance changes forneighboring nodes so as to accurately determine a touch location lyingbetween nodes by interpolation. Furthermore, the controller 114advantageously is designed to detect multiple distinct touches appliedto different portions of the touch device at the same time, or atoverlapping times. Thus, for example, if another finger 132 touches thetouch surface of the device 110 at touch location 133 simultaneouslywith the touch of finger 130, or if the respective touches at leasttemporally overlap, the controller is preferably capable of detectingthe positions 131, 133 of both such touches and providing such locationson a touch output 114 a. The number of distinct simultaneous ortemporally overlapping touches capable of being detected by controller114 is preferably not limited to 2, e.g., it may be 3, 4, or more,depending on the size of the electrode matrix.

As discussed further below, the controller 114 preferably employs avariety of circuit modules and components that enable it to rapidlydetermine the coupling capacitance at some or all of the nodes of theelectrode matrix. For example, the controller preferably includes atleast one signal generator or drive unit. The drive unit delivers adrive signal to one set of electrodes, referred to as drive electrodes.In the embodiment of FIG. 1, the column electrodes 116 a-e may be usedas drive electrodes, or the row electrodes 118 a-e may be so used. Thedrive signal is preferably delivered to one drive electrode at a time,e.g., in a scanned sequence from a first to a last drive electrode. Aseach such electrode is driven, the controller monitors the other set ofelectrodes, referred to as receive electrodes. The controller 114 mayinclude one or more sense units coupled to all of the receiveelectrodes. For each drive signal that is delivered to each driveelectrode, the sense unit(s) generate response signals for the pluralityof receive electrodes. Preferably, the sense unit(s) are designed suchthat each response signal comprises a differentiated representation ofthe drive signal. For example, if the drive signal is represented by afunction f(t), which may represent voltage as a function of time, thenthe response signal may be or comprise, at least approximately, afunction g(t), where g(t)=d f(t)/dt. In other words, g(t) is thederivative with respect to time of the drive signal f(t). Depending onthe design details of the circuitry used in the controller 114, theresponse signal may include: (1) g(t) alone; or (2) g(t) with a constantoffset (g(t)+a); or (3) g(t) with a multiplicative scaling factor(b*g(t)), the scaling factor capable of being positive or negative, andcapable of having a magnitude greater than 1, or less than 1 but greaterthan 0; or (4) combinations thereof, for example. In any case, anamplitude of the response signal is advantageously related to thecoupling capacitance between the drive electrode being driven and theparticular receive electrode being monitored. Of course, the amplitudeof g(t) is also proportional to the amplitude of the original functionf(t). Note that the amplitude of g(t) can be determined for a given nodeusing only a single pulse of a drive signal, if desired.

The controller may also include circuitry to identify and isolate theamplitude of the response signal. Exemplary circuit devices for thispurpose may include one or more peak detectors, sample/hold buffer,and/or low-pass filter, the selection of which may depend on the natureof the drive signal and the corresponding response signal. Thecontroller may also include one or more analog-to-digital converters(ADCs) to convert an analog amplitude to a digital format. One or moremultiplexers may also be used to avoid unnecessary duplication ofcircuit elements. Of course, the controller also preferably includes oneor more memory devices in which to store the measured amplitudes andassociated parameters, and a microprocessor to perform the necessarycalculations and control functions.

By measuring an amplitude of the response signal for each of the nodesin the electrode matrix, the controller can generate a matrix ofmeasured values related to the coupling capacitances for each of thenodes of the electrode matrix. These measured values can be compared toa similar matrix of previously obtained reference values in order todetermine which nodes, if any, have experienced a change in couplingcapacitance due to the presence of a touch.

Turning now to FIG. 2, we see there a schematic side view of a portionof a touch panel 210 for use in a touch device. The panel 210 includes afront layer 212, first electrode layer 214 comprising a first set ofelectrodes, insulating layer 216, second electrode layer 218 comprisinga second set of electrodes 218 a-e preferably orthogonal to the firstset of electrodes, and a rear layer 220. The exposed surface 212 a oflayer 212, or the exposed surface 220 a of layer 220, may be or comprisethe touch surface of the touch panel 210.

FIG. 3 a depicts a touch device 310 in which relevant controllercircuitry, such as drive and detection circuitry, is shown in thecontext of a touch panel 312 having one drive electrode 314 and onereceive electrode 316 capacitively coupled thereto via couplingcapacitance C_(c). The reader will understand that this is ageneralization of a touch panel in which drive electrode 314 may be oneof a plurality of drive electrodes, and receive electrode 316 likewisemay be one of a plurality of receive electrodes, arranged in a matrix onthe touch panel.

Indeed, in one specific embodiment of interest capable of use with atleast some of the touch measurement techniques described herein, thetouch panel may comprise a 40×64 (40 rows, 64 columns) matrix devicehaving a 19 inch diagonal rectangular viewing area with a 16:10 aspectratio. In this case, the electrodes may have a uniform spacing of about0.25 inches. Due to the size of this embodiment, the electrodes may havesignificant stray impedances associated therewith, e.g., a resistance of40K ohms for the row electrodes and 64K ohms for the column electrodes.For good human factors touch response, the response time to measure thecoupling capacitance at all 2,560 nodes of the matrix (40*64=2560) may,if desired, be made to be relatively fast, e.g., less than 20 or evenless than 10 milliseconds. If the row electrodes are used as the driveelectrodes and the column electrodes used as the receive electrodes, andif all of the column electrodes are sampled simultaneously, then the 40rows of electrodes have, for example, 20 msec (or 10 msec) to be scannedsequentially, for a time budget of 0.5 msec (or 0.25 msec) per rowelectrode (drive electrode).

The drive electrode 314 and receive electrode 316 of FIG. 3 a, which aredepicted by their electrical characteristics (in the form of lumpedcircuit element models) rather than by their physical characteristics,are representative of electrodes that may be found in a touch devicehaving a matrix smaller than 40×64, but this is not to be consideredlimiting. In this representative embodiment of FIG. 3 a, the seriesresistances R shown in the lumped circuit models may each have values of10K ohms, and the stray capacitances C shown in the lumped circuitmodels may each have values of 20 picofarads (pf), but of course thesevalues are not to be taken as limiting in any way. In thisrepresentative embodiment the coupling capacitance C_(c) is nominally 2pf, and the presence of a touch by a user's finger 318 at the nodebetween electrodes 314, 316 causes the coupling capacitance C_(c) todrop by about 25%, to a value of about 1.5 pf. Again, these values arenot to be taken as limiting.

In accordance with the controller described earlier, the touch device310 uses specific circuitry to interrogate the panel 312 so as todetermine the coupling capacitance C_(c) at each of the nodes of thepanel 312. In this regard, the reader will understand that thecontroller may determine the coupling capacitance by determining thevalue of a parameter that is indicative of, or responsive to, thecoupling capacitance, e.g., an amplitude of a response signal asmentioned above and described further below. To accomplish this task,the device 310 preferably includes: a low impedance drive unit 320coupled to the drive electrode 314; a sense unit 322 coupled to thereceive electrode 316, which, in combination with the couplingcapacitance, performs a differentiation on the drive signal supplied bythe drive unit; and an analog-to-digital converter (ADC) unit 324 thatconverts an amplitude of the response signal generated by the sense unit322 into a digital format. Depending on the nature of the drive signalsupplied by the drive unit 320 (and hence also on the nature of theresponse signal generated by the sense unit 322), the device 310 mayalso include a peak detection circuit 326 a which in this embodimentalso serves as a sample/hold buffer, and an associated reset circuit 326b operable to reset the peak detector. In most practical applicationsthe device 310 will also include a multiplexer between the signalgenerator 320 and the touch panel 312, so as to have the capability ofaddressing any one of a plurality of drive electrodes at a given time,as well as a multiplexer between the sense unit 322 (or between theoptional circuit 326 b) and the ADC unit 324, to allow a single ADC unitto rapidly sample the amplitudes associated with multiple receiveelectrodes, thus avoiding the expense of requiring one ADC unit for eachreceive electrode.

The drive unit 320 preferably is or includes a voltage source with aninternal impedance that is preferably low enough to maintain good signalintegrity, reduce injected noise, and/or maintain fast signal rise andfall times. The drive unit 320 provides a time-varying drive signal atan output thereof to the drive electrode 314. The drive signal mayconsist essentially of a single, isolated pulse, or it may comprise aplurality of such pulses or a train of pulses that form a continuous ACwaveform, or waveform packet, such as a sinusoidal wave, a square wave,a triangle wave, and so forth. In this regard, the term “pulse” is usedin a broad sense to refer to a distinctive signal variation and is notlimited to a rectangular shape of short duration and high amplitude. Ifrapid detection of touch(es) on the touch panel is desired, the drivesignal preferably includes only the smallest number of pulses necessaryto obtain a reliable measurement of the coupling capacitance at a givennode. This becomes particularly important for touch panels that havelarge electrode matrices, i.e., a large number of nodes to sense. Thepeak or maximum amplitude of the drive pulse(s) is preferably relativelyhigh, e.g., from 3 to 20 volts, to provide good signal-to-noise ratios.Though shown in FIG. 3 a as driving electrode 314 from only one end, insome embodiments drive unit 320 may be configured to drive electrode 314from both of its ends. This may be useful, for example, when electrode314 has high resistance (thus increased drive signal attenuation andsusceptibility to noise contamination), as may exist on large ITO-basedmatrix-type touch sensors.

The reader should keep in mind that there may be a distinction betweenthe drive signal provided at the output of drive unit 320, and the drivesignal being delivered to a particular drive electrode 314. Thedistinction becomes important when, for example, a multiplexer or otherswitching device is placed between the drive unit 320 and the touchpanel 312 in order to selectively couple the drive unit to a pluralityof drive electrodes, e.g., one at a time. In such a case, the drive unit320 may have at its output a continuous AC waveform, such as squarewave, triangle wave, or the like, yet by virtue of the switching actionof the multiplexer, only one pulse of such a waveform, or only a fewpulses, may be delivered to any given drive electrode at a time. Forexample, one pulse of a continuous AC waveform may be delivered to afirst drive electrode, the next pulse of the AC waveform may bedelivered to the next drive electrode, and so on until all driveelectrodes have been driven, whereupon the next pulse of the AC waveformis delivered again to the first drive electrode and so forth in arepeating cycle.

As will be explained further below in connection with FIGS. 4-6, theshape of the pulses used in the drive signal may have an impact on thechoice of detection/measurement electronics to be used in the device.Examples of useable pulse shapes include rectangle pulses, ramped pulses(whether symmetric or asymmetric), and sine wave (e.g., bell-shaped)pulses.

The drive unit 320 may if desired be programmable to provide differentpulses at different times. For example, if the drive unit is coupled toa plurality of drive electrodes through a multiplexer, the drive unitmay be programmed to provide different signal levels for different driveelectrodes to compensate for electrode-to-electrode variations in lineresistance and stray capacitance. For example, a drive electrodedisposed at a position that requires a long conduction length throughthe receive electrode(s) is beneficially driven with a higher amplitudedrive signal than a drive electrode disposed at a position that requiresa shorter conduction length, so as to compensate for losses associatedwith the receive electrodes. (For example, referring to the electrodematrix of FIG. 1, if row electrodes 118 a-e are the drive electrodes,then a drive signal on electrode 118 a is coupled through longer lengthsof the receive electrodes 116 a-e than a drive signal on electrode 118 edue to the placement of the control lines 126 proximate electrode 118e.) Providing different drive signal levels for different driveelectrodes in this way is particularly advantageous for large electrodematrices, because rather than programming a large number of detectioncircuits (corresponding to the number of receive electrodes) for lossesin the touch screen, only one drive signal is adjusted by a selectedamount, with drive signals delivered to different drive electrodes beingadjusted by differing amounts as appropriate.

The drive signal provided to the drive electrode 314 is capacitivelycoupled to receive electrode 316 via the coupling capacitance C_(c), thereceive electrode in turn being connected to sense unit 322. The senseunit 322 thus receives at an input thereof 322 a the drive signal (astransmitted by the electrodes 314, 316 and coupling capacitance C_(c)),and generates therefrom a response signal at an output 322 b.Preferably, the sense unit is designed so that the response signalincludes a differentiated representation of the drive signal, anamplitude of which is responsive to the coupling capacitance C_(c). Thatis, the response signal generated by the sense unit preferably includesin some form at least an approximation of the derivative with respect totime of the drive signal. For example, the response signal may includethe time derivative of the drive signal, or a version of such signalthat is inverted, amplified (including amplification less than 1),offset in voltage or amplitude, and/or offset in time, for example. Torepeat from the earlier discussion, if the drive signal delivered to thedrive electrode is represented by a function f(t), then the responsesignal may be or comprise, at least approximately, a function g(t),where g(t)=d f(t)/dt.

An exemplary circuit to perform such function is shown in FIG. 3 a. Theinput to such circuit, shown at 322 a, is the inverting input (−) of anoperational amplifier 322 c. The other input of the op amp, anon-inverting input (+), is set to a common reference level that can beoptimized for maximum signal range. In FIG. 3, this reference level isshown as ground potential for simplicity, but non-zero offset voltagescan also be used. A feedback resistor 322 d is connected between theoutput of the op amp at 322 b and the inverting input. When connected inthis way, the inverting input of the op amp 322 c, i.e., the input 322a, is maintained as a virtual ground summing point, and no signal isobserved at that point. This also means that the receive electrode 316is maintained at a constant voltage substantially equal to the voltageat which the non-inverting input of the op amp is held. The feedbackresistor 322 d can be selected to maximize signal level while keepingsignal distortion low, and can be otherwise set or adjusted as describedherein.

The op amp 322 c connected in this fashion, in combination with thecoupling capacitance C_(c), has the effect of producing a differentiatedrepresentation of the drive signal that is delivered to drive electrode314. In particular, the current I flowing through the feedback resistor322 d at any given time is given by:

I≈C _(c) *dV/dt,

where C_(c) is the coupling capacitance, V represents the time-varyingdrive signal delivered to the drive electrode, and dV/dt is thederivative with respect to time of V. Although this equation isnominally correct, the reader will understand that it does not take intoaccount various second order effects caused by, for example, parasiticresistance and capacitance of the electrodes being used, op ampcharacteristics and limitations, and the like, which can affect both themagnitude and the dynamic response of the current I. Nevertheless, thecurrent I, flowing through the feedback resistor, produces a voltagesignal at the output 322 b which corresponds to the response signaldiscussed above. Due to the direction of current flow through thefeedback resistor, this response signal is inverted insofar as apositive dV/dt (V increases with time) produces a negative voltage atoutput 322 b, and a negative dV/dt (V decreases with time) produces apositive voltage at output 322 b, with specific examples given below inconnection with FIGS. 4-6. This can be expressed as:

V _(RS) ≈−R _(f) *C _(c) *dV/dt,

where V_(RS) represents the response signal voltage at the output 322 bat any given time, and R_(f) is the resistance of feedback resistor 322d. Note that the amplitude (voltage) of the response signal is nominallyproportional to the coupling capacitance C_(c). Thus, since a touch atthe node of the electrodes 314, 318 reduces the coupling capacitanceC_(c), a measure of the peak amplitude or other characteristic amplitudeof the response signal provided by sense unit 322 can be analyzed todetermine the presence of a touch at that node.

In embodiments in which receive electrode 316 is one of a plurality ofreceive electrodes, it may be desirable to include a dedicated senseunit 322 for each receive electrode. Further, it may be advantageous toprovide different amounts of amplification (e.g., different feedbackresistor values for the different op amps) for the different sense unitsto compensate for signal losses in the touch screen that are differentfor different drive electrodes. For example, a receive electrodedisposed at a position that requires a long conduction length throughthe drive electrode(s) is beneficially provided with a greateramplification than a receive electrode disposed at a position thatrequires a shorter conduction length, so as to compensate for lossesassociated with the drive electrodes. (For example, referring to theelectrode matrix of FIG. 1, if row electrodes 116 a-e are the receiveelectrodes, then a signal received from electrode 116 a is coupledthrough longer lengths of the drive electrodes 118 a-e than a signalreceived from electrode 116 e due to the placement of the control lines128 proximate electrode 116 e.) Providing different amounts ofamplification for different receive electrodes in this way isparticularly advantageous for large electrode matrices, because it canreduce the need to program a large number of detection circuits(corresponding to the number of receive electrodes) for losses in thetouch screen.

As mentioned above, device 310 may also include peak detection circuit326 a which in this embodiment also serves as a sample/hold buffer, andan associated reset circuit 326 b operable to reset the peak detector.These circuit elements can be used in cases where the peak amplitude ofthe response signal generated by the sense unit 322 is to be used as ameasure of the coupling capacitance C_(c). Such cases can includeembodiments in which the response signal provided by the sense unit 322is highly transient, e.g., in cases where one or more rectangle pulsesare used for the drive signal (see e.g. FIG. 4 a below). In such cases,the peak detector 326 a operates to maintain the peak amplitude of theresponse signal for a relatively long time to allow reliable samplingand conversion to a digital value by the ADC 324. In embodiments havinga plurality of receive electrodes, a single ADC may be cyclicallycoupled to the detection circuitry of each receive electrode, requiringeach detection circuit to maintain the measurement voltage for anextended period of time. After the measurement is made by the ADC 324,the peak detector can be reset by operation of reset circuit 326 b sothat a new peak value can be measured in a subsequent cycle.

The basic operation of the diode/capacitor combination depicted for peakdetector 326 a, including its ability to maintain the peak voltage foran extended period without discharging the capacitor through the senseunit 322, will be apparent to the person of ordinary skill in the art,with no further explanation being necessary. Likewise, the basicoperation of the reset circuit 326 b, responding to a suitable resetcontrol signal provided at contact 326 c, will be apparent to the personof ordinary skill in the art. Note that other known electronic devicescapable of carrying out one or more functions of the described senseunit, peak detector, sample/hold buffer, and/or reset circuit, whetherin hardware, software, or combinations thereof, are fully contemplatedherein.

As mentioned previously, the ADC 324 is preferably provided to convertthe amplitude value associated with the response signal to a digitalformat for use with digital components such as a microprocessor forfurther processing. The ADC may be of any suitable design, e.g., it maycomprise a high speed successive approximation register (SAR) and/or asigma-delta type converter.

With regard to further processing of the measured amplitude value of agiven node, the measured amplitude value can be stored in a memoryregister. If desired, multiple such values associated with the givennode may be stored and averaged, e.g. for noise reduction purposes.Furthermore, the measured amplitude value is preferably compared to areference value in order to determine if a reduction of the couplingcapacitance has occurred, i.e., if some amount of touch is present atthe given node. Such comparison may involve subtraction of the measuredvalue from the reference value, for example. In embodiments involving alarge touch matrix containing many nodes, the measured values for all ofthe nodes can be stored in memory, and individually compared torespective reference values in order to determine if some amount oftouch is present at each node. By analyzing the comparison data, thepositions of multiple temporally overlapping touches, if present on thetouch surface, can be determined. The number of temporally overlappingtouches capable of being detected may be limited only by the dimensionsof the electrode grid in the touch panel and the speed of thedrive/detection circuitry. In exemplary embodiments, interpolation isperformed for differences detected for neighboring nodes so as toaccurately determine a touch location lying between nodes.

FIG. 3 b depicts touch device 348 which is similar to touch device 310shown in FIG. 3 a, except that it includes voltage source 349 as aninput to the differentiating amplifier that is part of sense unit 322.This voltage input may be configured as needed to bring circuit outputinto a sensing range for the ADC. For example, some ADCs have sensingranges from 0.5V to +3V. The peak of the sense unit 322 output signalshould be within this range to digitize the voltage accurately. Voltagesource 349 (or gain, in the context of sense unit 322) can be fixed atone voltage for all receiver electrodes, or it can be adjusted forparticular receive electrodes. In some embodiments, differing voltagesare provided to sense units in groups of 4-10 receive electrodes using aresistor ladder network. In some embodiments, gain is set to compensatefor signal drop off due to resistance on the driven electrodes.

FIG. 3 c depicts touch device 350 which is similar to touch device 310shown in FIG. 3 a, but containing additional circuitry that in someembodiments may better accommodate noise from displays such as LCDdisplays. LCD addressing frequencies are generally near or overlappingthe frequencies used by controller 114 to interface with touch panel112. This results in noise on the receiver electrodes which may show upas a common mode signal. A differential amplifier may be used toeliminate this common mode signal. The circuit shown in FIG. 3 c adds adifferential amplifier 352 and additional peak detection circuit 351(configured to detect peaks of negative voltage), and an additionalreset circuit 353.

Turning now to FIG. 4 a, we see there a voltage vs. time graph of aparticular drive signal 410 and a corresponding voltage vs. time graphof a (modeled) response signal 412 generated by a sense unit of the typedepicted in FIG. 3 a. For purposes of the model, the electroniccharacteristics of the drive electrode, receive electrode, and couplingcapacitance (including the effect of a touch thereon, i.e., decreasingthe capacitance from 2.0 pf to 1.5 pf) were assumed to be as describedabove in connection with the representative embodiment of FIG. 3 a.Furthermore, the feedback resistor 322 d for the op amp 322 c wasassumed to be on the order of 2M ohms.

The drive signal 410 is seen to be a square wave, containing a series ofrectangle pulses 411 a, 411 c, 411 e, . . . 411 k. This entire signalwas assumed to be delivered to a particular drive electrode, although inmany embodiments a smaller number of pulses, e.g. only one or two, maybe delivered to a given drive electrode at a given time, after which oneor more pulses may be delivered to a different drive electrode, and soon. The response signal 412 generated by the sense unit is seen tocomprise a plurality of impulse pulses 413 a-l, two for each rectanglepulse 411 a, as one would expect for a differentiated square wave. Thus,for example, the drive pulse 411 a yields a negative-going impulse pulse413 a associated with the positive-going transition (left side) of therectangle pulse, and a positive-going impulse pulse 413 b associatedwith the negative-going transition (right side) of the rectangle pulse.The impulse pulses are rounded as a result of the op amp signalbandwidth and the RC filter effects of the touch screen. Despite thesedeviations from an ideal derivative with respect to time of signal 410,the response signal 412 can be considered to comprise a differentiatedrepresentation of the drive signal.

As shown, the drive pulses 411 a, 411 c, 411 e, . . . 411 k, all havethe same amplitude, although pulses of differing amplitude can also bedelivered as explained above. However, despite the common amplitude ofthe drive pulses, the impulse pulses 413 a-g occurring in the timeperiod 412 a are seen to have a first peak amplitude, and impulse pulses413 h-l occurring in the time period 412 b are seen to have a secondpeak amplitude less than the first peak amplitude. This is because themodel introduced a change in coupling capacitance C_(c) at a point intime after impulse pulse 413 g and before impulse pulse 413 h, thechange corresponding to a transition from a non-touch condition (C_(c)=2pf) to a touch condition (C_(c)=1.5 pf). The reduced peak amplitude ofthe impulse pulses during time period 412 b can be readily measured andassociated with a touch event at the applicable node.

The transient nature of the impulse pulses 413 a-l make themparticularly suited for use with a peak detector and sample/hold bufferas described in connection with FIG. 3, so that an accurate measurementof the peak amplitude can be obtained and sampled by the ADC.

FIG. 4 b depicts graphs showing representative waveforms from anembodiment that includes sequential driving of driven electrodes.Waveforms 430, 431, and 432 are representative of pulsed signals duringa period of time, t, on three separate (possibly adjacent one another)driven electrodes (a first, second, and third row on a matrix-typesensor, for example). Waveforms 433, 434, and 435 are representative ofdifferentiated output resulting from the pulsed signals on threeseparate receive electrodes (columns on a matrix-type sensor, forexample) during the same time period. Note that each receive electrode(column) has a similar response profile. The driven electrodescorresponding to waveforms 432, 431, and 431 are driven sequentially.After each an electrode is driven (represented by any individual ones ofwaveforms 430, 431, or 432), a voltage representative of peak amplitudewill be available in the peak detect circuit associated with eachreceive electrode (column) as described above in connection with FIG. 3.Thus, after each driven electrode is driven (row), the resultant voltageon the peak detect circuit for all receive electrodes (columns) issampled, then the associated peak detect circuit reset, then the nextsequential driven electrode is driven (and so on). In this way, eachnode in the matrix-type capacitive touch sensor can be individuallyaddressed and sampled.

FIG. 5 a depicts a pair of graphs similar to those of FIG. 4 a, and forthe same electronic configuration of drive electrode, receive electrode,coupling capacitance, and sense unit, but for a different drive signalshape. The drive signal 510 of FIG. 5 a includes ramped pulses 511 a,511 c, 511 e, . . . 511 i, so that the resultant response signal 512includes rectangle pulses 513 a-j. The rectangle pulses predicted by themodel exhibited near-vertical hi/lo transitions with slightly roundedcorners, which have been redrawn as vertical lines and sharp corners forsimplicity. The rise and fall times of the rectangle pulses are limitedby the RC transmission line in the drive and receive electrodes beingused. The drive pulses 511 a, etc. are characterized by a symmetricalramp shape, with the first half of each pulse having a positive-goingslope and the second half having a negative-going slope of the samemagnitude. This symmetry is also then carried over to the responsesignal 512, where negative-going pulses 513 a, 513 c, and so forth aresubstantially balanced by positive-going pulses 513 b, 513 d, and so on.Similar to the description of FIG. 4 a, the model introduces a change incoupling capacitance C_(c) at a point in time after rectangle pulse 513e and before rectangle pulse 513 f, i.e., in the transition from timeperiod 512 a to time period 512 b, the change corresponding to atransition from a non-touch condition (C_(c)=2 pf) to a touch condition(C_(c)=1.5 pf). The reduced amplitude of the response signal pulsesoccurring during time period 412 b can be readily measured andassociated with a touch event at the applicable node. A feature of FIG.5 a worth noting is the relatively steady-state characteristic (over thetime scale of given pulse) of the response signal 512 at each plateau ofeach pulse 513 a-j, where the “plateau” of a negative-going pulse 513 a,513 c, and so on is understood to be the “bottom” of the pulse shaperather than the “top” as with pulses 513 b, d, and so forth. Thissteady-state characteristic is a consequence of the drive pulses havinga constant slope over a substantial portion of the drive pulses, i.e., aramped shape. In some embodiments, the touch device designer may wish totake advantage of this steady-state characteristic so as to eliminateunnecessary circuit items and reduce cost. In particular, since theresponse signal itself maintains a substantially constant amplitude (theplateau of a pulse) over the time scale of the pulse, and since thisconstant amplitude is indicative of or responsive to the couplingcapacitance C_(c), the peak detector, sample/hold buffer, and resetcircuit described in connection with FIG. 3 a may no longer be necessaryand may be eliminated from the system, provided the time scale of thesteady-state characteristic is long enough for the ADC to sample andmeasure the amplitude. If desired, for noise-reduction, the responsesignal generated by the sense unit in such cases can be sent through alow-pass filter whose cutoff frequency is selected to substantiallymaintain the same overall fidelity or shape as the unfiltered pulseswhile filtering out higher frequency noise. The output of such a filter,i.e., the filtered response signal, may then be supplied to the ADC. Ofcourse, in some cases it may be desirable to keep the peak detector,sample/hold buffer, and reset circuit, whether or not the low-passfilter is utilized, for ramp-type drive pulses.

If desired, a rectifying circuit can be used in touch device embodimentsthat produce positive- and negative-going pulses in the response signal,see e.g. signal 412 of FIG. 4 a and signal 512 of FIG. 5 a. Therectification of these signals may have corresponding benefits for othercircuit functions, such as peak detection and analog-to-digitalconversion. In the case of signal 512 of FIG. 5 a, a rectified versionof that signal advantageously maintains a steady-state voltage levelsubstantially continuously (ignoring transient effects due to op amplimitations and RC transmission line effects) as a result of thesymmetry of the respective signals.

FIG. 5 b depicts pairs of graphs showing representative waveforms fromembodiments that include sequential driving of driven electrodes,similar to FIG. 4 b, except using a different type of driven waveform.Waveforms 760, 761, and 762 are representative driven triangle pulsesignals during a time period, t, on three separate (possibly adjacentone another) driven electrodes (a first, second, and third row on amatrix-type sensor, for example). Waveforms 763, 764, and 765 arerespective resultant waveforms as would be seen on receive electrodes(for example, columns) during the same time period.

Turning now to FIG. 6 a, the pair of graphs there are similar to thoseof FIGS. 5 a and 4 a, and assume the same electronic configuration ofdrive electrode, receive electrode, coupling capacitance, and senseunit, but a yet another drive signal shape is used. The drive signal 610of FIG. 6 b includes ramped pulses 611 a-e, which yield the resultantresponse signal 612 having substantially rectangle pulses 613 a-e.Unlike the symmetrical ramp shapes of FIG. 5 a, ramped pulses 611 a-eare asymmetrical so as to maximize the fraction of the pulse time usedby the ramp. This ramp maximization, however, results in a rapidlow-to-high transition on one side of each drive pulse, which produces anegative-going impulse pulse bounding each rectangle pulse of theresponse signal 612. In spite of the resulting deviations from perfectrectangularity, the pulses 613 a-e are nevertheless substantiallyrectangular, insofar as they maintain a relatively constant amplitudeplateau between two relatively steep high-to-low transitions. As such,and in a fashion analogous to signal 512 of FIG. 5 a, the pulses ofsignal 612 include a steady-state characteristic as a consequence of thedrive pulses having a constant slope over a substantial portion of thedrive pulses, i.e., a ramped shape. The touch device designer may thusagain wish to take advantage of this steady-state characteristic byeliminating the peak detector, sample/hold buffer, and reset circuit,provided the time scale of the steady-state characteristic is longenough for the ADC to sample and measure the amplitude. A low-passfilter may also be added to the circuit design as described above.

FIG. 6 b depicts a pair of graphs showing representative waveforms fromembodiments that include sequential driving of driven electrodes,similar to FIG. 4 b and FIG. 5 b, except using a different type ofdriven waveform. Waveforms 750, 751, and 752 are representative drivenramped pulse signals during a time period, t, on three separate(possibly adjacent one another) driven electrodes (a first, second, andthird row on a matrix-type sensor, for example). Waveforms 753, 754, and755 (FIG. 7 b) and 763, 764, and 765 (FIG. 7 c) are respective resultantwaveforms as would be seen on receive electrodes (for example, columns)during the same time period.

Turning now to FIG. 7, we see there a voltage vs. time graph of a pulseddrive signal 807 and a corresponding voltage vs. time graph of a(modeled) first response signal 801 and second response signal 802 aswould be output generated by sense unit 322 and differential amplifier352, respectively, of the circuit depicted in FIG. 3 c. For purposes ofthe model, the electronic characteristics of the drive electrode,receive electrode, and coupling capacitance (including the effect of atouch thereon, i.e., decreasing the capacitance from 2.0 pf to 1.5 pf)were assumed to be as described above in connection with therepresentative embodiment of FIG. 3 a.

First response signal 801 is the modeled output from sense unit 322. Itincludes a sinusoidal form indicative of a common mode signal similar tothat which might be received as noise from an LCD panel. Response signal802 is the respective modeled output from differential amplifier 352(shown for the purposes of illustration as a short-dashed line; theactual output would be a solid line). The output from differentialamplifier 352 is in effect the sum of the pulses (shown not to scale forillustrative purposes). The individual pulses on FIG. 7 (803 a . . . d,804 e, f, g) have the same profile as pulses 413 a . . . k in FIG. 4 a,but they appear differently in FIG. 7 due to scaling. The first negativepulse (803 a) is peak detected and summed on the inverting input of theamplifier giving the first step on response signal 802 (step 805 a). Thepositive pulse (804 e) is then peak detected and summed on thenon-inverting input on the amplifier, giving the sum of both thepositive and negative peaks at the output (step 805 b). Neithersucceeding pulses nor the common mode signal substantially effect thevoltage level of response signal 802 after step 805 b. A touch may besensed by measuring a first voltage sample represented by waveform 802after a series of pulses (that is, after the voltage has reached aplateau defined by step 805 b), resetting peak detectors using resetcircuits 353 and 326 b (FIG. 3 c), and then measuring a second voltagesample using the same or a similar process, and so forth. In certainembodiments, changes to these sample voltages, relative to somethreshold, are indicative of a touch.

FIG. 8 is a schematic view of a touch device 710 that includes a touchpanel 712 having a 4×8 matrix of capacitively coupled electrodes, andvarious circuit components that can be used to detect multiplesimultaneous touches on the touch panel. The electrode matrix includes atop electrode array comprising parallel drive electrodes a, b, c, and d.Also included is a lower array comprising parallel receive electrodesE1, E2, E3, E4, E5, E6, E7, and E8. The top electrode array and thelower electrode array are arranged to be orthogonal to one another. Thecapacitive coupling between each pair of orthogonal electrodes, referredto above for a given node as the coupling capacitance C_(c), is labeledfor the various nodes of the matrix as C1 a, C2 a, C3 a, C4 a, C1 b, C2b, and C3 b, etc., through C8 d as shown, the values of which may all beapproximately equal in an untouched state but which decrease when atouch is applied as described previously. Also depicted in the figure isthe capacitance between the various receive electrodes and ground(C1-C8) and between the various drive electrodes and ground (a′ throughd′).

The 32 nodes of this matrix, i.e., the mutual capacitances or couplingcapacitances associated therewith, are monitored by circuitry asdescribed with respect to FIG. 3 a: drive unit 714; multiplexer 716;sense units S1-S8; optional peak detectors P1-P8, which may alsofunction as sample/hold buffers; multiplexer 718; as well as ADC 720;and controller 722, all connected as shown with suitable conductivetraces or wires (except that connections between controller 722 and eachof the peak detectors P1-P7 are omitted from the drawing for ease ofillustration).

In operation, controller 722 causes drive unit 714 to generate a drivesignal comprising one or more drive pulses, which are delivered to driveelectrode a by operation of multiplexer 716. The drive signal couples toeach of receive electrodes E1-E8 via their respective mutualcapacitances with drive electrode a. The coupled signal causes the senseunits S1-S8 to simultaneously, or substantially simultaneously, generateresponse signals for each of the receive electrodes. Thus, at this pointin time in the operation of device 710, the drive signal being deliveredto drive electrode a (which may include, for example, a maximum of 5, 4,3, or 2 drive pulses, or may have only one drive pulse) is causing senseunit S1 to generate a response signal whose amplitude is indicative ofcoupling capacitance C1 a for the node E1/a, and sense unit S2 togenerate a response signal whose amplitude is indicative of couplingcapacitance C2 a for the node E2/a, etc., and so on for the other senseunits S3-S8 corresponding to nodes E3/a through E8/a, all at the sametime. If the response signals are of a highly transient nature, e.g. aswith signal 412 of FIG. 4 a, then peak detectors P1-P8 may be providedto detect the peak amplitudes of the respective response signalsprovided by sense units S1-S8, and optionally to sample and hold thoseamplitudes at the outputs thereof which are provided to the multiplexer718. Alternatively, if the response signals have a significantsteady-state characteristic, e.g. if they are in the form of one or morerectangle pulses as with signals 512 and 612 described above, then thepeak detectors may be replaced with low-pass filters, or the peakdetectors may simply be omitted so that the outputs of the sense unitsfeed directly into the multiplexer 718. In either case, while thecharacteristic amplitude signals (e.g. peak amplitude or averageamplitude of the response signals) are being delivered to themultiplexer 718, the controller 722 rapidly cycles the multiplexer 718so that the ADC 720 first couples to peak detector P1 (if present, or toa low-pass filter, or to S1, for example) to measure the characteristicamplitude associated with node E1/a, then couples to peak detector P2 tomeasure the characteristic amplitude associated with node E2/a, and soforth, lastly coupling to peak detector P8 to measure the characteristicamplitude associated with node E8/a. As these characteristic amplitudesare measured, the values are stored in the controller 722. If the peakdetectors include sample/hold buffers, the controller resets them afterthe measurements are made.

In the next phase of operation, the controller 722 cycles themultiplexer 714 to couple the drive unit 714 to drive electrode b, andcauses the drive unit to generate another drive signal that againcomprises one or more drive pulses, now delivered to electrode b. Thedrive signal delivered to electrode b may be the same or different fromthat delivered previously to electrode a. For example, for reasonsrelating to touch panel losses explained above, the drive signaldelivered to electrode b may have a smaller amplitude than thatdelivered to electrode a, due to electrode b's closer proximity to theends of sense electrodes E1-E8 from which the response signals arederived (and thus lower losses). In any case, the drive signal deliveredto electrode b causes sense unit S1 to generate a response signal whoseamplitude is indicative of coupling capacitance C1 b for the node E1/b,and sense unit S2 to generate a response signal whose amplitude isindicative of coupling capacitance C2 b for the node E2/b, etc., and soon for the other sense units S3-S8 corresponding to nodes E3/b throughE8/b, all at the same time. The presence or absence of peak detectorsP1-P8, or of sample/hold buffers, or of low-pass filters discussed abovein connection with the first phase of operation is equally applicablehere. In any case, while the characteristic amplitude signals (e.g. peakamplitude or average amplitude of the response signals) are beingdelivered to the multiplexer 718, the controller 722 rapidly cycles themultiplexer 718 so that the ADC 720 first couples to peak detector P1(if present, or to a low-pass filter, or to S1, for example) to measurethe characteristic amplitude associated with node E1/b, then couples topeak detector P2 to measure the characteristic amplitude associated withnode E2/b, and so forth, lastly coupling to peak detector P8 to measurethe characteristic amplitude associated with node E8/b. As thesecharacteristic amplitudes are measured, the values are stored in thecontroller 722. If the peak detectors include sample/hold buffers, thecontroller resets them after the measurements are made.

Two more phases of operation then follow in similar fashion, wherein adrive signal is delivered to electrode c and the characteristicamplitudes associated with nodes E1/c through E8/c, are measured andstored, and then a drive signal is delivered to electrode d and thecharacteristic amplitudes associated with nodes E1/d through E8/d, aremeasured and stored.

At this point, characteristic amplitudes of all of the nodes of thetouch matrix have been measured and stored within a very shorttimeframe, e.g., in some cases less than 20 msec or less than 10 msec,for example. The controller 722 may then compare these amplitudes withreference amplitudes for each of the nodes to obtain comparison values(e.g., difference values) for each node. If the reference amplitudes arerepresentative of a non-touch condition, then a difference value of zerofor a given node is indicative of “no touch” occurring at such node. Onthe other hand, a significant difference value is representative of atouch (which may include a partial touch) at the node. The controller722 may employ interpolation techniques in the event that neighboringnodes exhibit significant difference values, as mentioned above.

Unless otherwise indicated, all numbers expressing quantities,measurement of properties, and so forth used in the specification andclaims are to be understood as being modified by the term “about”.Accordingly, unless indicated to the contrary, the numerical parametersset forth in the specification and claims are approximations that canvary depending on the desired properties sought to be obtained by thoseskilled in the art utilizing the teachings of the present application.Not as an attempt to limit the application of the doctrine ofequivalents to the scope of the claims, each numerical parameter shouldat least be construed in light of the number of reported significantdigits and by applying ordinary rounding techniques. Notwithstandingthat the numerical ranges and parameters setting forth the broad scopeof the invention are approximations, to the extent any numerical valuesare set forth in specific examples described herein, they are reportedas precisely as reasonably possible. Any numerical value, however, maywell contain errors associated with testing or measurement limitations.

Various modifications and alterations of this invention will be apparentto those skilled in the art without departing from the spirit and scopeof this invention, and it should be understood that this invention isnot limited to the illustrative embodiments set forth herein. Forexample, the reader should assume that features of one disclosedembodiment can also be applied to all other disclosed embodiments unlessotherwise indicated. It should also be understood that all U.S. patents,patent application publications, and other patent and non-patentdocuments referred to herein are incorporated by reference, to theextent they do not contradict the foregoing disclosure.

1. A touch-sensitive apparatus, comprising: a panel comprising a touchsurface and a plurality of electrodes defining an electrode matrix, theplurality of electrodes comprising a plurality of drive electrodes and aplurality of receive electrodes, each drive electrode being capacitivelycoupled to each receive electrode at a respective node of the matrix,the panel being configured such that a touch on the touch surfaceproximate a given one of the nodes changes a coupling capacitancebetween the drive electrode and the receive electrode associated withthe given node; a drive unit configured to generate a drive signal andto deliver the drive signal to the drive electrodes one at a time; asense unit configured to generate, for each drive signal delivered toeach drive electrode, response signals for the plurality of receiveelectrodes, each of the response signals comprising a differentiatedrepresentation of the drive signal, an amplitude of each of the responsesignals being responsive to the coupling capacitance at the associatednode; and a measurement unit configured to measure the amplitude of eachof the response signals for each of the nodes, and to determinetherefrom the positions of multiple temporally overlapping touches, ifpresent, on the touch surface.
 2. The apparatus of claim 1, wherein thedrive unit includes a drive signal generator and a multiplexer, thedrive signal generator being selectively couplable to a given one of thedrive electrodes through the multiplexer.
 3. The apparatus of claim 1,wherein the sense unit includes, for each of the receive electrodes, anoperational amplifier having an inverting input coupled to therespective receive electrode.
 4. The apparatus of claim 1, wherein thesense unit is further configured to maintain the receive electrodes at afixed voltage.
 5. The apparatus of claim 1, wherein the drive signalcomprises a rectangle pulse.
 6. The apparatus of claim 1, wherein thesense unit includes, for each of the receive electrodes, a peak detectorconfigured to provide a peak detector output representative of a maximumamplitude of the respective response signal.
 7. The apparatus of claim6, wherein each peak detector comprises a sample/hold buffer.
 8. Theapparatus of claim 6, wherein each peak detector comprises a diodecoupled to a capacitor.
 9. The apparatus of claim 8, wherein the senseunit includes, for each of the receive electrodes, a reset switchcoupled to the respective capacitor and configured to discharge therespective capacitor in response to a reset signal.
 10. The apparatus ofclaim 1, wherein the measurement unit comprises an analog-to-digitalconverter (ADC) and a multiplexer, the ADC coupling to the sense unitthrough the multiplexer.
 11. The apparatus of claim 1, wherein the drivesignal comprises a plurality of sequential pulses and each responsesignal comprises a corresponding plurality of response pulses, andwherein the measurement unit is configured to measure for each responsesignal an amplitude representative of amplitudes of the plurality ofresponse pulses.
 12. The apparatus of claim 11, wherein the measurementunit is configured to measure for each response signal a maximum one ofthe amplitudes of the plurality of response pulses.
 13. The apparatus ofclaim 1, wherein the drive signal comprises a ramped pulse.
 14. Theapparatus of claim 13, wherein each response signal comprises arectangle pulse.
 15. The apparatus of claim 14, wherein the measurementunit comprises a low pass filter to smooth a plateau of the rectanglepulse.
 16. The apparatus of claim 14, wherein the measurement unitcomprises an analog-to-digital converter (ADC) and is adapted to coupleeach response signal to the ADC without passing the response signalsthrough any peak detector.
 17. The apparatus of claim 1, wherein thedrive unit is configured to deliver a first drive signal to a firstdrive electrode and a second drive signal to a second drive electrode,and wherein the first drive signal has a signal amplitude that differsfrom that of the second drive signal.
 18. The apparatus of claim 1,wherein the sense unit includes a first sense unit coupled to a firstreceive electrode and a second sense unit coupled to a second receiveelectrode, and wherein the first sense unit has an amplificationassociated therewith that differs from an amplification associated withthe second sense unit.
 19. The apparatus of claim 1, wherein themeasurement unit additionally includes a differential amplifierconfigured to reduce or eliminate common mode noise.
 20. Atouch-sensitive apparatus, comprising: a panel comprising a touchsurface and a plurality of electrodes defining an electrode matrix, theelectrode matrix being configured such that a touch on the touch surfaceproximate a given node of the matrix changes a coupling capacitancebetween two of the electrodes; a drive unit coupled to the electrodematrix and configured to generate a drive signal comprising one or moreramped pulses; a sense unit coupled to the electrode matrix andconfigured to generate, in response to the drive signal, at least oneresponse signal that includes one or more rectangle pulses, an amplitudeof the at least one response signal being responsive to a touch on thetouch surface.